1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) receiving filter to be used in a radio frequency (RF) filter that is used in compact mobile communication devices such as cellular phones and so forth.
2. Description of Related Art
Recently the development of compact-size and light-weight mobile communication device terminals such as cellular phones and so forth has been progressed. Along with the development, compact size and high performance of parts to be used therein are required, and so is the development of SAW elements.
Since a SAW separator is a device that will greatly contribute to making an RF portion compact, there is a great demand for its practical applications.
An RF portion shown in FIG. 7 comprises an antenna 101, an ANT terminal 102 that is connected to the above antenna, a transmitting filter 103 and a separator line 107 that are respectively connected to the above ANT terminal, a transmitting terminal (Tx terminal) 104 that is connected to the above transmitting filter, an electric power amplifier 105 that is connected to the above transmitting terminal, a receiving filter 108 that is connected to the above separator line, and a receiving terminal (Rx terminal) 109 that is connected to the above receiving filter.
The SAW separator 110 comprises the transmitting filter 103, the receiving filter 108 and the separator line 107, and such a SAW separator are required to have its characteristics including compact size, low insertion loss of a pass band, large attenuation in an attenuation band, and so on.
Since the SAW separator 110 is arranged at output line of the electric power amplifier 105 at the final stage, electric power load from 1 to 2 W is imposed on the transmitting filter 103, while electric power load around 1 W is imposed on the receiving filter 108, therefore, excellent electric power-resisting property is required to the SAW separator.
Conventionally, a dielectric wave separator has been used in such a field and apt to make volume large, which has been a problem with the prior art.
In the transmitting filter 103 and the receiving filter 108, used in general is a ladder type filter as shown in FIG. 8. The filter is equipped with series arm SAW resonators S1, S2 and S3, and parallel arm SAW resonators P1, P2, P3 and P4. The series arm SAW resonator S1 at a first stage has interdigitated width of 50 .mu.m and number of pairs of 100, the series arm SAW resonator S2 at a second stage has interdigitated width of 50 .mu.m and number of pairs of 100, and the series arm SAW resonator S3 at a third stage has interdigitated width of 50 .mu.m and number of pairs of 100. The parallel arm SAW resonator P1 at a first stage has interdigitated width of 70 .mu.m and number of pairs of 70, the parallel arm SAW resonator P2 at a second stage has interdigitated width of 110 .mu.m and number of pairs of 80, the parallel arm SAW resonator P3 at a third stage has interdigitated width of 110 .mu.m and number of pairs 80, and the parallel arm SAW resonator P4 at a fourth stage has interdigitated width of 70 .mu.m and number of pairs of 70.
FIG. 9 is a diagram showing a symbol of each of the respective SAW resonators (S1-S3, P1-P4) in FIG. 8, and FIG. 10 is a diagram showing a lumped parameter equivalent circuit therein. These respective resonators are arranged with their comb-shape electrodes or interdigital electrodes mutually engaged as shown in FIG. 11.
In FIG. 11, reference numerals 10a and 10b show comb-shape type electrodes or interdigital electrodes, and they are engaged with each other, and reference numeral 12 shows a pair of finger electrodes, while the code D shows a interdigitated width of the finger electrodes.
Usually, each of the resonators has finite Q, and there will occur electric power loss that decreases this Q value. A resistance component that causes this electric power loss generates heat when electric power is supplied to the resonator, and this heat causes damage on electrodes. This fact is known to those skilled in the art. Especially in the case of a receiving filter of a SAW separator, attention should be given to heat generating at electrodes of a parallel arm at a first stage.
In the circuit structure shown in FIG. 8, in order to prevent the above damage, a method wherein the number of pairs of the input stage resonator S1 of series arm is made larger than the number of each of other series arm resonators S2 and S3 is known in the field. (Refer to, for example, Japanese Unexamined Patent Publication (TOKKAIHEI) No.6(1994)-29779.)
However, in the above receiving filter of conventional SAW separator, no measures are taken on the number of pairs in parallel arm resonators. Accordingly, when electric power is applied to a parallel arm resonator at a first stage, heat is generated or released in electrodes at the first stage, and the heat causes damage on electrodes, which has been a problem to the conventional SAW separator.
For example, a receiving filter of the structure shown in FIG. 8 has been employed in a SAW separator. When electric power is applied to the SAW separator, a transmitting filter and a receiving filter employed in the SAW separator are broken.
A ladder type filter shown in FIG. 8 is well known as one wherein comb-shape electrodes are melted off owing to heat generated by current flowing through series arms (S1, S2, S3) and parallel arms (P1, P2, P3, P4). Especially in operating a cellular phone, parallel arms in a receiving filter of a SAW separator generate more heat than series arms. The parallel arm at a first stage generates heat most among the respective parallel arms, whereby electrodes of parallel arms are broken, and consequently the receiving filter is broken. Accordingly, measures to enhance electric power-resisting properties of parallel arms (P1, P2, P3, P4) have been required.
In view of the above circumstances, an object of the present invention is to provide a receiving filter of a SAW separator with high resistance to electric power and with high performance by preventing temperature thereof from rising.